Advancements in the recording industry include reproducing sound from a multiple channel sound system, such as reproducing sound from a surround sound system. Typically, surround sound or two-channel stereo recordings are recorded and then processed to be reproduced over loudspeakers, which limits the quality of such recordings when reproduced over headphones. For example, stereo recordings are usually meant to be reproduced over loudspeakers, instead of being played back over headphones. This results in the stereo panorama appearing on line in between the ears or inside a listener's head, which can be an unnatural and fatiguing listening experience.
To resolve the issues of reproducing sound over headphones, designers have derived stereo and surround sound enhancement systems for headphones; however, for the most part these enhancement systems have introduced unwanted effects such as unwanted coloration, resonance, reverberation, and/or distortion of timbre or sound source angle and/or position.
Sound color or timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices, musical instruments, and sound systems (e.g., loudspeakers and headphones). The physical characteristics of sound that determine the perception of timbre include spectrum and envelope. Generally, timbre is what makes a particular musical sound different from another, even when they have the same pitch and loudness.
U.S. Patent Application Publication No. 2013/0236023 to Horbach, describes a system for headphone equalization using an “equal loudness method”, which filters the unwanted effects from the audio signals provided by headphones. The system includes a stored set of predetermined tone burst reference signals and a stored set of predetermined tone burst test signals that form a range of frequencies used in a user specific audio test to develop a headphone correction filter. A predetermined tone burst reference signal and a predetermined tone burst test signal may intermittently and sequentially drive a transducer included in the headphone. A loudness of the predetermined tone burst reference signal may be fixed and a loudness of the predetermined tone burst test signal may be variable with a gain setting. The gain setting may be used to generate the headphone correction filter.